1. Field of the Invention
The present invention relates to a new process for the preparation of biuret polyisocyanates, to the compounds obtained by this process and to their use as polyisocyanate components in two-component polyurethane lacquers.
2. Description of the Prior Art
Aliphatic polyisocyanates, in particular biuret polyisocyanates based on 1,6-diisocyanatohexane, have achieved a position of great technical importance for the preparation of light fast and extremely weather resistant lacquers with maximum gloss retention. They are used in combination with hydroxyl group-containing polyesters or polyacrylates. These two-component polyurethane lacquers are used predominantly for lacquering relatively rigid substrates such as metals and wood and are distinguished by their great hardness and very firm adherence.
The chemical basis for these lacquers and coatings is described inter alia in "Lackunstharze" by Hans Wagner and Hans Friedrich Sarx, Carl Hanser Verlag, Munich 1971, pages 153 to 173.
The preparation of biuret polyisocyanates, i.e. the reaction of diisocyanates such as, in particular, 1,6-diisocyanatohexane with biuretizing agents, has been disclosed in numerous prior publications. The following are examples of processes described in the literature:
Synthesis from diisocyanates and water, optionally in the presence of catalysts; see DE-PS 1,110,394, DE-OS 1,668,377, DE-OS 2,308,015, GB-PS 889,050, GB-PS 1,399,228 and DD-PS 140,744, PA1 Synthesis from diisocyanates and water in the presence of a solvent or solvent mixture, see DE-OS 2,808,801 and DE-OS 3,030,655; PA1 Synthesis from diisocyanates and water, the water being reacted in the form of steam, see DE-OS 2,918,739; PA1 Synthesis from diisocyanates and amine/alcohol mixtures, see DE-OS 2,654,745; PA1 Synthesis from diisocyanates and .omega.,.omega.'-diaminopolyethers, see DE-OS 1,570,632 and DE-AS 1,215,365; PA1 Synthesis from diisocyanates and substituted ureas, see DE-PS 1,101,394 and DE-AS 1,227,004; PA1 Synthesis from diisocyanates and tertiary alcohols, optionally in the presence of catalysts, see DE-AS 1,543,178, DE-AS 1,931,055 and DE-OS 2,308,015; PA1 Synthesis from diisocyanates and formic acid, see DE-PS 1,174,760, DE-OS 2,308,015 and DE-OS 2,437,130; PA1 Synthesis from diisocyanates and aldoximes, see DE-OS 3,007,679; PA1 Synthesis from diisocyanates and trisubstituted acetic acids and/or anhydrides and optionally water, see DE-OS 3,403,277 and 3,403,278; PA1 Synthesis from diisocyanates and diamines using special smooth jet nozzles for the preparation of the reaction mixture, see EP-A-3505; PA1 Use of 1,6-diisocyanatohexane in combination with (cyclo)aliphatic diamines, optionally with the addition of low molecular weight polyhydroxyl compounds (molecular weight below 500) at reaction temperatures above 250.degree. C., see earlier German Patent Application P 37 00 209.0. PA1 (a) 1,6-diisocyanatohexane with subequivalent quantities of PA1 (b) biuretizing agents based on water and/or organic compounds which react with isocyanate groups to form biuret groups, PA1 (c) at least one diol containing ester groups and having an average molecular weight of about 350 to 950 is added to the reaction mixture in a quantity of about 1 to 50% by weight, based on the weight of the 1,6-diisocyanatohexane, the diol reacting with part of the isocyanate groups present to form urethane groups, PA1 1,6-diisocyanatohexane (a) is always introduced in such an excess that at the end of the reaction the reaction mixture still contains at least 10% by weight, preferably about 35 to 70% by weight of free 1,6-diisocyanatohexane, based on the total quantity of mixture not including any inert solvents used, PA1 the quantity of biuretizing agent (b) is kept low enough that not more than 50 mole %, preferably about 10 to 30 mole % of the isocyanate groups of 1,6-diisocyanatohexane (a) undergo reaction to form biuret groups, the calculation being based on the assumption 15 that the products of the biuretization reaction are entirely N,N',N"-trisubstituted biurets and PA1 the molar ratio of biuret groups to urethane groups in the products of the process is about 20:1 to 0.2:1, preferably about 5:1 to 0.5:1.
Synthesis from diisocyanates and amines, see DE-PS 1,165,580, DE-PS 1,174,759, DE-OS 1,568,017, DE-OS 1,963,190, DE-OS 2,010,887, DE-OS 2,261,065, DE-AS 2,438,258, US-P 3,824,266, DE-AS 2,609,995, DE-OS 2,830,103, DE-PS 883,504, GB-PS 1,263,609, see also Angew. Chem. 72, page 1002;
The two-component polyurethane lacquers known in the art often result in highly cross-linked lacquer coats which have insufficient elasticity for flexible substrates. In the motor car industry in particular flexible plastic parts are increasingly used for improved safety. These flexible molded parts (bumpers, spoilers, frames for outside mirrors, and the like) are relatively large and therefore contribute considerably to the external appearance of the vehicle. It is therefore necessary to lacquer these parts. Added to this is the fact that the surfaces of the plastics are deteriorated by the weather and must therefore be protected. Elastic lacquerings are also required for less elastic plastic parts to prevent mechanical damage to the parts. For example, hard but tough thermoplasts must be lacquered with highly elastic, extremely resistant lacquers so that mechanical damage or other external influences will not produce cracks in the lacquer film which would be propagated into the compact plastic. The lacquers for such parts, especially the top coat lacquers, must therefore conform to standards much higher than those required for ordinary lacquers.
A partial solution to these problems was the development of hydroxyl-polyesters and polyacrylates which can be worked up into elastic lacquer films by virtue of their structure. It was not possible to overcome all of the difficulties by these means. The lacquer films produced are in many cases not sufficiently hard and not sufficiently cross-linked and/or have too little chemical resistance.
It was therefore an object of the present invention to provide new polyisocyanates which when combined with known polyols to form two-component polyurethane lacquers would satisfy the above-mentioned special requirements and would in particular be optimally suited for lacquering elastic plastic parts.
This problem has been solved by providing the process according to the invention described below for the preparation of urethane-modified biuret polyisocyanates.